diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
index d18f082b4..a337c1c69 100644
--- a/include/nlohmann/json.hpp
+++ b/include/nlohmann/json.hpp
@@ -1304,10 +1304,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    reference set_parent(reference j, std::size_t old_capacity = -1)
+    reference set_parent(reference j, std::size_t old_capacity = std::size_t(-1))
     {
 #if JSON_DIAGNOSTICS
-        if (old_capacity != -1)
+        if (old_capacity != std::size_t(-1))
         {
             // see https://github.com/nlohmann/json/issues/2838
             JSON_ASSERT(type() == value_t::array);
@@ -1322,6 +1322,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         j.m_parent = this;
 #else
         static_cast<void>(j);
+        static_cast<void>(old_capacity);
 #endif
         return j;
     }
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 3b5a7a75d..429964dd7 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -18339,10 +18339,10 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         return it;
     }
 
-    reference set_parent(reference j, std::size_t old_capacity = -1)
+    reference set_parent(reference j, std::size_t old_capacity = std::size_t(-1))
     {
 #if JSON_DIAGNOSTICS
-        if (old_capacity != -1)
+        if (old_capacity != std::size_t(-1))
         {
             // see https://github.com/nlohmann/json/issues/2838
             JSON_ASSERT(type() == value_t::array);
@@ -18357,6 +18357,7 @@ class basic_json // NOLINT(cppcoreguidelines-special-member-functions,hicpp-spec
         j.m_parent = this;
 #else
         static_cast<void>(j);
+        static_cast<void>(old_capacity);
 #endif
         return j;
     }